High Levels of Natural Perchlorate in a Desert Ecosystem

USGS scientists collect soil, plant, and atmospheric-material samples to determine processes that control the accumulation, retention, and release of natural perchlorate in a terrestrial ecosystem. Photo credit: Brian Andraski, USGS.

Naturally formed perchlorate falls from the atmosphere and accumulates in the soil of a Nevada desert at rates several times greater than previously thought, according to research by U.S. Geological Survey (USGS) and Texas Tech University scientists published in the Journal of Environmental Quality. The study is the first to document how natural perchlorate is delivered from the atmosphere to the soil and then cycled between soil and plants in a terrestrial ecosystem. Perchlorate has emerged as an environmental contaminant of concern in drinking water and food. The study results can help land and water-resource managers assess risks posed by perchlorate to ecosystems and human health, and to the potential movement of natural perchlorate into groundwater.

Perchlorate is a manufactured chemical used in rocket fuel and fireworks, but it also forms naturally in the atmosphere and falls to the land surface in precipitation and other airborne materials, including dust. While perchlorate contamination near former military and industrial sites has been well documented, natural background levels of the chemical are poorly understood. The team of scientists quantified natural levels of perchlorate in desert soil, plants, and atmospheric deposition in the Amargosa Desert, and they identified what controls the chemical's cycling and accumulation. Knowing these background levels and how perchlorate participates in soil-plant-atmosphere processes will help other scientists and land managers identify, interpret, and treat sites contaminated by human uses of perchlorate.

Shallow soils (0–30 centimeters [cm]) in the study area, near the USGS Amargosa Desert Research Site, contained a high level of perchlorate (14,490 milligrams per hectare [mg/ha]). There is no federal drinking water standard for perchlorate, but if that amount of highly soluble perchlorate were flushed to groundwater, it would be sufficient to contaminate roughly a quarter million gallons of water per acre, based on California's maximum contaminant level.

The study identified the creosote bush canopy as a previously unrecognized reservoir for perchlorate (1,310 mg/ha). Creosote bush is the dominant shrub in North American warm deserts and is browsed by many small mammals, whereas other desert shrubs, such as shadscale and burrobush, are eaten by livestock and wildlife. Thus, study results show that vegetation plays a key role in regulating the accumulation, retention, and release of perchlorate across the land surface, and suggests that the transfer of perchlorate from soil and plants to higher organisms may contribute to perchlorate exposure in a desert ecosystem.

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The perchlorate atmospheric deposition rate (343 mg/ha/year) measured by the team is about 10 times greater than previously reported for the southwestern United States. Atmospheric deposition occurs both from rain—which washes compounds out of the atmosphere—and from the settling out of dry airborne materials. The previously reported rates were based on samples from atmospheric deposition network collectors that normally exclude dry deposition; this study showed that dry deposition is an important contributor of perchlorate to the land surface.